Brown the beef in a large skillet over medium heat, stirring occasionally, about 5 minutes. Transfer to a medium bowl (leave the drippings in the pan).
Cook the scallions, pine nuts, if using, and garlic in the drippings for 3 minutes. Stir in the zest, season with salt and pepper and mix with the meat. Preheat the oven to 400 degrees .
Cut a 1/4-inch-thick lengthwise slice from each zucchini. Using a melon baller or teaspoon, scoop out and discard the seeds, creating four 1/4-inch-thick "boats".
Arrange the zucchini in a medium baking dish and fill each with some of the beef mixture. Pour 1/2 cup chicken broth into the bottom of the dish, cover with foil and bake until the zucchini are tender, about 30 minutes.
Bring the remaining 1 1/4 cups broth to a boil in a small saucepan, add the EVOO and stir in the couscous. Cover, turn off the heat and let stand for 5 minutes. Fluff with a fork; keep warm.
Divide the couscous and zucchini boats among plates. Top with the yogurt, tomatoes and cucumber. Sprinkle with the parsley.